<!DOCTYPE HTML>
<html>
<head>
<style>
body {
  margin: 0;
}
#main {
  width: 500px;
  height: 100px;
  opacity: 0.999;
  background: lime;
  border: 1px solid black;
}
.censor {
  position: absolute;
  width: 1px;
  height: 1px;
  background: cyan;
}
</style>
</head>
<body>
<div id="main"></div>
<div class="censor" style="left:0; top:0;"></div>
<div class="censor" style="left:501px; top:0;"></div>
<div class="censor" style="left:0; top:101px;"></div>
<div class="censor" style="left:501px; top:101px;"></div>
</body>
</html>
